Once I knew I had a winning ticket for the race, I did what any programmer would do when getting started.

No, I didn’t make the t-shirt. I wrote a simulation.

Step #1 was to get the profile of the race course. Google Earth has nice support for generating a profile from a path:

Next was figuring out how to simulate the race. For every meter down the course, the profile lets me calculate the change in elevation, and thus the change in potential energy. Once I have this, and the mass of the vehicle, and the resistance due to wheels and air drag, and the moment of inertia of the wheels, then it was a simple matter of mapping from delta PE to delta KE:

And then I could turn these equations into code:

Finally, I could generate results with different weights, and graph them.

The sad result was that more weight == better results. Though I imagine rolling resistance isn’t linear with weight, as the “footprint” of the tread expands under increased load. Plus there’s the issue of being able to stop at the bottom of the hill…

I’ve been keeping a collection of emails sent by my Dad, as he works in his shop to get a disk brake attached to our (small) wheels. I think it’s pretty amazing that he’s got the energy and persistence (at 85!) to keep working away at a problem until he solves it.

Two days ago:

I have been chasing my tail for several days trying to get the brake discs to rotate without ‘runout’ when installed with the wheel on my support brackets. I have gotten as close as +/- .005″; not good enough.

Each time I lapped the face the disc bolts to, and reinstalled the disc, I got different results. I tried adding shims to the bearing outer races – no luck.

Finally, tonight I removed one of the spacers from between the bearings and measured the length at 4 points with a micrometer. 1.0303, 1.0302, 1.0295, 1.0288. That calculates to an angle of 0.1836 degrees.

Now I wonder how to true up the sleeve. I don’t know which end is off. And I don’t know if my lathe and chuck are accurate enough. I can also chuck it in my milling chuck. If I grind off a little I will need to add a hard stainless shim type washer to compensate and keep the length to 1.030. I have shim stock in various thicknesses.

Yesterday was:

1) I checked out the lathe; it is pretty accurate.
2) I trued up the sleeves by chucking in the lathe and grinding with my Dremel. One end was off on each of them.
3) Brake disc run-out, and the wheel still wobbles!!

And today, victory!

Today it finally dawned on me that the top hat we made to mount the brake disc to the wheel really forms a backbone for both. The disc is pretty flexible; the wheels are made from fiberglass reinforced plastic which has a low modulus of elasticity. In between is the rigid metal top hat.

By adjusting the bolts to the wheel, I have the one wheel running less than +/- .002 instead of +/- .020. Now I am working on the face of the hat against the disc. If need be I can shim and get it accurate. The gap in the brake clamp is .090 with a little drag on the feeler gauge. The disc is .070 thick, leaving less than .020 total.

June 16th is the second annual Nevada City Adult Soapbox Derby, where grown men (and some women) raise money for Pioneer Park, by spending ridiculous amounts of time building gravity-powered vehicles.

Here’s the local hospital entry from last year:

This year, since I’ve got so much free time, I decided to put together a team. Thus was born the “Tube of Terror”, sponsored by Scale Unlimited. With the aid of some people who actually know how how to design and build things, I’m hoping we’ll be competitive in the speed category.